Android scroll performance is really bad IF text selection is enabled

Problem

There was a regression in Android scroll speed. On 8/25 the scroll speed was acceptable, and on the latest master is really slow.

Solution

Run a git bisect and figure out what broke it

cc @quinthar

[quinthar]

Actually going to go LOW, as I don’t even know that this is important for genuine day to day usage. I have a lot of Android apps that can’t select text

[AndrewGable]

tl;dr: The contractor has recreated the issue and is looking into fixes for RN core.

Here is the current conversation:

[9/10/2020 3:38:24 PM] Andrew Gable: Hi Nazim Gadzhiagayev - We have actually started to implement our own React Native application in house, but we would love to have you help us with bugs and issues we run into as you are a React Native expert!

The current bug we are running into is we have a <ScrollView> with quite a few <Text> components in it (~300-1000 text elements). On Android when we set the <Text> component’s property selectable to true, we get very poor scroll performance. When it is set to false, we get great scroll performance at 60 FPS.

Would you mind looking into this for us? If so, could you:

1. Make sure you can recreate this bug (If not, we will share more details)
2. Keep us updated on your progress on this bug daily

Thank you, I look forward from hearing back soon.

• Andrew

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[9/10/2020 3:44:14 PM] David Barrett: To expand on this, ideally you would solve the bug in the most “correct” way possible – not just hack around it, but contribute a bugfix into the mainline react native code. Let’s figure out how easy/hard it is to engage with and contribute to the core RN maintainers.

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[9/11/2020 11:49:42 AM] Nazim Gadzhiagayev: Hey, i was able to reproduce the issue. (First suggestion) Ideally we should prefer FlatList over ScrollView as it uses VirtualizedList under the hood and has better performance. But i noticed that you can’t use [selectable] as it doesn’t work on Android. I have spent couple hours on investigating RN code but haven’t founf any relevant solution yet. I will update you as I progress. Thanks.

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[9/11/2020 2:23:37 PM] Andrew Gable: Thank you for the update Nazim-

We would like to continue to use ScrollView as it has great performance on web, with acceptable performance on iOS/Android. So please do continue to update us on any improvements you can make on ScrollView.

• Andrew